portal: warn against a dot in the Claude connector name (breaks tool loading)

src/intervalsicu_mcp_ui/templates/account.html

@@ -31,4 +31,10 @@
   <h2>Connect Claude</h2>
   <p>Add a custom connector in Claude pointing at:</p>
   <p><code>{{ mcp_url }}</code></p>
+  <p class="banner warn">
+    <strong>Name the connector without a dot.</strong> Use something like
+    <code>intervals</code> &mdash; <em>not</em> <code>intervals.icu</code>.
+    Claude Desktop and mobile currently don&rsquo;t support a <code>.</code> in the
+    connector name, and it will stop the tools from loading.
+  </p>
 {% endblock %}